1. Field of the Invention
The invention relates to a process for preparing copolymers in which vinyl esters and ethylene are present, while reclaiming the residual unreacted ethylene, via free-radical-initiated polymerization of one or more vinyl esters, ethylene, and also, where appropriate, other ethylenically unsaturated monomers copolymerizable therewith, at a pressure of from 5 to 100 bar abs. by emulsion or suspension polymerization, in an aqueous medium
2. Background Art
Polymers based on vinyl ester and ethylene, and also, where appropriate, vinyl chloride and (meth)acrylate monomers, are used especially in the form of their aqueous dispersions or of their water-redispersible polymer powders in a wide variety of applications, for example as coating compositions or adhesives for a correspondingly wide variety of substrates. Protective colloids or low-molecular-weight surface-active compounds are used to stabilize these polymers. The protective colloids generally used comprise polyvinyl alcohols.
High conversion levels are the state of the art in large-scale industrial polymerization processes. For example, polymerization of the polymers is usually complete to the extent that the residual monomer content is  less than 0.1% by weight, preferably  less than 0.05% by weight, and in the case of vinyl chloride  less than 0.01% by weight. However, these high conversions are based only on the monomers which are liquid under polymerization conditions. Ethylene is a monomer which is gaseous under polymerization conditions, and does not follow these guidelines. Firstly, it polymerizes more slowly than other monomers such as vinyl acetate, and secondly it is largely present in the gas phase and is inaccessible under the usual conditions of emulsion polymerization. Specifically, the polymerization process proceeds only in the liquid phase, with involvement of ethylene which has dissolved in water, in monomer, or in a particle.
For reasons of cost-effectiveness, the intention is that large-scale industrial polymerization processes should reach completion in the shortest possible times, but this inevitably means that it is impossible to consume all of the ethylene used. The polymerization process is usually interrupted, and the reaction mixture depressurized, when the residual ethylene gas content is less than 5% by weight, preferably less than 2% by weight. The depressurization procedure encompasses the transfer of the reaction mixture (polymer dispersion+residual gas) from a pressure reactor into an unpressurized reactor, with removal of residual ethylene. Further monomer removal from the resultant latex then takes place in a known manner. However, the excess of ethylene is discarded, generally via combustion.
This prior-art procedure has the disadvantage of low cost-effectiveness, due to poor monomer utilization and high disposal costs. Reuse of the resultant ethylene is inhibited by the fact that the residual gas would have to be recompressed in advance to a high pressure ( greater than 80 bar). This is an energy-intensive process which, on grounds of cost-effectiveness, inhibits recycling, because the residual gas also has to be purified in a complicated process prior to recompression, in order, for example, to avoid the occurrence of pressure surges during the compression process.
The prior art discloses various processes for reclaiming residual ethylene. WO-A 01/00559 describes processes for reclaiming ethylene from the ethylene-containing inert gas stream arising during vinyl acetate preparation. The ethylene-containing inert gas is absorbed in vinyl acetate, the ethylene is liberated via depressurization in a vacuum vessel, and is then recompressed for reuse. In another process described, the ethylene-containing inert gas stream is absorbed in acetic acid and is brought into contact with ethylene-containing residual gas in a stripping column, the ethylene reclaimed at the column head, and recycled into the vinyl acetate synthesis process. EP-A 127253 relates to the removal of residual monomer in the gas-phase polymerization of copolymers of ethylene and higher olefins. For this, the solid copolymer is freed from the higher olefins at reduced pressure and then treated with inert-gas-free reactor gas, which is finally returned to the polymerization process.
It was an object of the present invention to provide a process which permits cost-effective reuse of the residual gas. This and other objects are achieved by depressurizing an ethylene-containing reaction mixture, compressing a gas phase obtained thereby to a modestly increased pressure, absorbing the ethylene-containing compressed gas into one or more vinyl ester monomers, and employing the ethylene-containing vinyl ester(s) in the same or a further polymerization process.